1. Field of the Invention
The invention relates to an optical semiconductor device and a method of fabricating the same, and more particularly to a waveguide type optical semiconductor device having a function of spot-size conversion and a method of fabricating the same.
2. Description of the Related Art
With the recent development of an optical access system typical of xe2x80x9cfiber to the home (FTTH)xe2x80x9d, a semiconductor laser module used for optical communication is desirable fabricated at a lower cost.
One of major factors for keeping a fabrication cost of a semiconductor laser module high is a packaging cost necessary for optically coupling a laser diode to an optical fiber. Hence, an attention has been recently paid to a spot-size converted semiconductor laser diode which readily accomplishes higher optical coupling between a laser diode and an optical fiber. Herein, a spot-size converted semiconductor laser diode (SSC-LD) is a laser diode which enlarges a spot-size at a plane through which a laser beam leaves, to thereby keep a beam divergence angle small. A smaller beam divergence angle would reduce lights radiated into a free space to thereby ensure a higher optical coupling efficiency for optically coupling a laser diode to an optical fiber. In other words, provision of a semiconductor laser diode with a function of a lens would make it no longer necessary for a semiconductor laser diode to have an optical lens system which was absolutely necessary for a conventional semiconductor laser module. Thus, A semiconductor laser module could be fabricated at lower costs.
In order to enlarge a spot-size at a plane at which laser beams leave a laser diode, it would be necessary to make an optical confinement factor small at the above-mentioned plane in an optical waveguide to thereby enlarge an optical field. Specifically, an optical waveguide is designed to include a spot-size converting portion having a thickness smaller than other portions. A spot-size conversion (SSC) structure like this is useful for all of waveguide type optical semiconductor devices such as a an optical semiconductor modulator, an optical semiconductor amplifier and a waveguide pin photo diode as well as a semiconductor laser diode.
For instance, one of SSC-LDs has been suggested by Y. Tohmori et al. in ELECTRONICS LETTERS, Jun. 22nd, 1995, Vol. 31, No. 13, pp. 1069-1070 (hereinafter, referred to as first prior art). FIGS. 1A to 1E are cross-sectional views of a laser diode showing respective steps of a method of fabricating a laser diode in accordance with the first prior art.
As illustrated in FIG. 1A, a laser active layer is formed on an InP substrate 401. The laser active layer comprises a first separate confinement hetero-structure (SCH) layer 402, a strained multi-quantum well (MQW) structure 403, and a second SCH layer 404, and each of them are successively epitaxially grown by metal-organic vapor phase epitaxy (hereinafter, referred to simply as MOVPE) growth method.
Then, a SiNx layer 405 is formed on the laser active layer. Then, a portion which would make an SSC portion is etched until the InP substrate 401 appears with the SiNx layer 405 used as a mask. Then, as illustrated in FIG. 1B, an SSC structure comprising a 1.1 xcexcm-bandgap wavelength InGaAsP layer 406 is selectively grown to thereby form a butt-joint.
Then, the SiNx layer 405 is removed, followed by growth of a p-InP clad layer 407 and a p-cap layer 408 all over the product, as illustrated in FIG. 1C.
Then, an SiNx stripe mask 409 is formed partially on the p-cap layer 408, and thereafter the product is etched until a certain depth of the InP substrate 401 with the SiNx stripe mask 409 used as a mask to thereby form a high-mesa structure, as illustrated in FIG. 1D.
Then, the SiNx stripe mask 409 is removed only in the SSC portion, followed by growth of a Fe-doped highly resistive InP layer 410, as illustrated in FIG. 1E. The thus fabricated laser diode has a 300 xcexcm-long laser active layer region and a 300 xcexcm-long SSC region.
In the above-mentioned method of fabricating a laser diode in accordance with the first prior art, it is necessary to repeatedly carry out complicated steps of selective etching and selective re-growth, and it is also necessary to complete a waveguide by forming a butt-joint. Thus, the first prior art has a problem that it is difficult to fabricate a laser diode with a high fabrication yield.
Another example of SSC-LD has been suggested by T. Yamamoto in ELECTRONICS LETTERS, Dec. 7th, 1995, Vol. 31, No. 25, pp. 2178-2179 (hereinafter, referred to as second prior art), wherein a multi-quantum well (MQW) structure having different thicknesses and band-gap energies between a laser active layer region and an SSC region is formed by a single selective growth. Hereinbelow is explained the second prior art with reference to FIGS. 2A to 2D.
First, a pair of dielectric masks 502 having a width in the range of tens of micrometers to multi-hundreds of micrometers is formed on an n-InP substrate 501 with the masks 502 being spaced away from each other by 10-20 xcexcm, as illustrated in FIG. 2A.
Then, an n-InP clad layer 503, a strained MQW structure 504, and a p-InP clad layer 505 are selectively grown on the n-InP substrate 501 by MOVPE, as illustrated in FIG. 2B. In this selective growth of the layers 503, 505 and the structure 504, enhancement in a growth rate and increase in an In incorporation rate occur in a region sandwiched between the masks 502 due to vapor phase lateral diffusion of source materials. As a result, a thickness of MQW is enhanced and further a band-gap wavelength is made longer in the region sandwiched between the masks 502 in comparison with other region not sandwiched between the masks 502. Accordingly, the region sandwiched between the masks 502 makes a laser active layer, and the other region not sandwiched between the masks 502 makes an SSC region.
Then, after removal of the dielectric masks 502, a dielectric stripe mask 506 is formed over the selectively grown layers. Thereafter, the product is mesa-etched so that the laser active layer has a width of 1.2 xcexcm, as illustrated in FIG. 2C.
Then, a p-InP current block layer 507 and an n-InP current block layer 508 are grown all over the product. Then, after removal of the dielectric stripe mask 506, a p-InP second clad layer 509 and a cap layer 510 are grown over the n-InP current block layer 508, as illustrated in FIG. 2D. The thus formed laser active layer region is 300 xcexcm long, and the SSC region is 200 xcexcm long.
Still another example of a laser diode has been suggested by M. Wada et al. in ELECTRONICS LETTERS, Nov. 23rd, 1995, Vol. 31, No. 24, pp. 2102-2104. There has been suggested laser diodes monolithically integrated with spot-size converters operating at 1.3 xcexcm and having an almost circular far-field pattern and a xe2x88x921.3 dB butt-coupling-loss-to-fiber with wide alignment tolerance. However, the overall device length is 450 xcexcm.
In the above-mentioned first and second prior art, the SSC regions do not have an optical gain, because they are formed merely for enlarging a spot of laser oscillation lights. Accordingly, the first and second prior art are inferior to an ordinary semiconductor laser diode having no SSC region with respect to increasing of a threshold current and degrading performance at high temperature, because the SSC region causes optical losses.
In addition, a device yield per a unit area or per a wafer would be reduced in the above-mentioned conventional SSC-LDs, because they have to be fabricated longer by a length of the SSC region. Specifically, the laser diode in accordance with the first prior art includes the 300 xcexcm long laser active layer region and the 300 xcexcm long SSC region, and hence is totally 600 xcexcm long. The laser diode in accordance with the second prior art includes the 300 xcexcm long laser active layer region and the 200 xcexcm long SSC region, and hence is totally 500 xcexcm long. An ordinary laser diode having no SSC region is 300 xcexcm long. Thus, a yield per a wafer for fabricating devices is reduced by about 40-50% in the first and second prior art in comparison with the conventional laser diodes.
Moreover, the first prior art requires to carry out complicated fabrication steps of repeated selective etching of semiconductor layers and selective re-growth. In addition, since a butt-joint having a problem in repeatability is introduced into a waveguide, the first prior art has a problem in controllability and repeatability, resulting in difficulty in fabricating a semiconductor diode with a high yield and with high repeatability.
Since the second prior art employs mesa-etching/re-growth steps of semiconductor layers for forming an optical waveguide, the second prior art has the same problem as that of the first prior art. Namely, the second prior art has a problem in controllability and repeatability, resulting in difficulty in fabricating a semiconductor diode with a high yield and with high repeatability.
In view of the foregoing problems of the prior art, it is an object of the present invention to provide an optical semiconductor device capable of having an optical gain to laser oscillation wavelengths to thereby eliminate optical losses in an SSC region without being fabricated longer than a conventional optical semiconductor device, and also capable of achieving a lower threshold value characteristic and high temperature performance with a higher yield per a wafer.
In one aspect, there is provided an optical semiconductor device including an optical waveguide structure having a quantum well layer and an optical confinement layer as a core layer, the core layer having a thickness varying in a lengthwise direction of the optical waveguide to thereby have a function of spot-size conversion, the quantum well layer being designed to have a band-gap energy which is constant within xc2x130 meV in the direction.
There is further provided an optical semiconductor device including an optical waveguide structure having a quantum well layer and an optical confinement layer as a core layer, the core layer having a thickness varying in a lengthwise direction of the optical waveguide to thereby have a function of spot-size conversion, the quantum well layer being designed to have a thickness which is constant within xc2x132% in the direction.
It is preferable that the quantum well layer has a constant thickness and the optical confinement layer has a thickness varying in the direction, in which the optical confinement layer may have a thickness gradually reducing toward an end thereof through which a light leaves the optical confinement layer. It is preferable that the optical confinement layer has a first thickness smaller than that of the quantum well layer at the end thereof, but has a second thickness greater than that of the quantum well layer at the other end thereof, in which case a ratio of the second thickness to the first thickness is preferably equal to or greater than 2.
It is preferable that the quantum well layer has a constant thickness in the direction, and the optical confinement layer has a thickness which is maximum at the center in the direction.
The optical semiconductor device may serve as a semiconductor laser diode or an optical amplifier having the above-mentioned core layer as an active layer, in which case optical gain is obtained in the direction. As an alternative, the optical semiconductor device may serve as a electro-absorption type optical modulator or a waveguide type pin photo-diode, in which case the core layer may serve as an optical-absorption layer in the direction.
As mentioned earlier, the optical semiconductor device in accordance with the present invention has a quantum well structure in the core layer, and has a band-gap substantially constant in a lengthwise direction of the waveguide. Specifically, in a certain embodiment, the well layer has a thickness constant in the lengthwise direction of the waveguide, and the optical confinement and/or barrier layers have a thickness varying in the direction. Hereinbelow, an allowable range in which the band-gap and the thickness of the well layer may vary is explained with reference to FIGS. 3 and 4.
FIG. 3 shows an optical gain spectrum obtained when carriers of 1.50xc3x971018 cmxe2x88x923 are introduced into a 1.3 xcexcm-band quantum well semiconductor laser. An optical gain coefficient is positive when a photon energy (a photon wavelength is 1.30 xcexcm) is equal to 950 meVxc2x130 meV. That is, an optical gain can be obtained when photon energy is in the range of 920 meV to 980 meV.
FIG. 4 illustrates a relation between a band-gap wavelength and a thickness of a well layer in a quantum well structure. A range of a well layer thickness allowing xc2x130 meV tolerance around the band-gap wavelength of 1.30 xcexcm is a range of 3.7 nm to 7.2 nm, namely 5.45 nm +32%. Thus, the band-gap is necessary to be constant with xc2x130 meV tolerance, and the well layer thickness is necessary to be constant with xc2x132% tolerance.
In another aspect of the present invention, there is provided a method of fabricating an optical semiconductor device including an optical waveguide having a thickness varying in a lengthwise direction thereof, including the steps of (a) forming a pair of dielectric masks on a compound semiconductor substrate, the masks including a portion where a width thereof varies in the direction, (b) epitaxially growing a lower optical confinement layer by metal-organic vapor phase epitaxy (MOVPE) selective growth, (c) forming a quantum well structure by epitaxially growing a quantum well layer or a plurality of quantum well layers with barrier layers sandwiched between the quantum well layers by metal-organic vapor phase epitaxy (MOVPE) selective growth, and (d) epitaxially growing an upper optical confinement layer by metal-organic vapor phase epitaxy (MOVPE) selective growth, the quantum well layer(s) being grown in the step (c) under a growth pressure lower than that of the lower and upper optical confinement layers in the steps (b) and (d).
It is preferable that the barrier layers are grown in the step (c) under a growth pressure equal to or greater than that of the quantum well layers. The barrier layers are grown in the step (c) under a pressure preferably equal to or greater than 100 hPa, more preferably 200 hPa.
The quantum well layer(s) is(are) grown in the step (c) under a growth pressure preferably equal to or smaller than 40 hPa, more preferably 30 hPa.
The lower and upper optical confinement layers are grown in the steps (b) and (d) under a growth pressure preferably equal to or greater than 100 hPa, more preferably 200 hPa.
There is further provided a method of fabricating an optical semiconductor device including an optical waveguide having a thickness varying in a lengthwise direction thereof, including the steps of (a) forming a pair of dielectric masks on a compound semiconductor substrate, the masks including a portion where a width thereof varies in the direction, (b) epitaxially growing a lower optical confinement layer by metal-organic vapor phase epitaxy (MOVPE) selective growth, (c) forming a quantum well structure by epitaxially growing a quantum well layer or a plurality of quantum well layers with barrier layers sandwiched between the quantum well layers by metal-organic vapor phase epitaxy (MOVPE) selective growth, and (d) epitaxially growing an upper optical confinement layer by metal-organic vapor phase epitaxy (MOVPE) selective growth, the quantum well layer(s) being grown in the step (c) employing tertiarybutylarsine (TBA) and tertiarybutylphosphine (TBP) having a V/III ratio equal to or greater than 50.
It is preferable that the lower and upper optical confinement layers are grown in the steps (b) and (d) employing arsine and phosphine as group V source materials, or employing tertiarybutylarsine (TBA) and tertiarybutylphosphine (TBP) having a V/III ratio equal to or smaller than 5. It is also preferable that the barrier layers are grown in the step (c) employing arsine and phosphine as group V source materials, or employing tertiarybutylarsine (TBA) and tertiarybutylphosphine (TBP) having a V/III ratio equal to or smaller than 5.
FIG. 5 illustrates a mask pattern for MOVPE selective growth. Hereinafter, the principle of the present invention is explained with reference to FIG. 5. A pair of stripe masks 12 is formed on an InP substrate 11 having a (100) plane as a principal plane so that a gap formed between the masks 12 is oriented in the [011] direction. Each of the masks 12 has a width of Wm, and the masks 12 are spaced away from each other by a distance Wg=1.5 xcexcm. A selective growth layer 13 is formed by MOVPE selective growth in an area indicated with Wg and formed between the masks 12.
It is known to those skilled in the art that if the mask width Wm were increased, a thickness or a growth rate of the selective growth layer 13 would be increased, and as a result, In composition would be increased in the case that the selective growth layer 13 is formed of InGaAs or InGaAsP. Hereinbelow are explained the dependency of a growth rate and composition of a selective growth layer on a mask width to be used for selective growth, and the dependency of the same on growth conditions.
There are two mechanisms for an increase in a growth rate, The first one is that source materials supplied onto a mask reach a growth region by surface-migration on a mask, and as a result, a growth rate is increased and becomes greater than a growth rate in the case of non-selective growth, namely growth in unmasked region. The second one is vapor phase lateral diffusion of source materials caused by a concentration gradient produced in vapor phase. Specifically, source materials are consumed in growth region, whereas source materials are not consumed in a mask region. As a result, a concentration gradient is produced in vapor phase. The concentration gradient causes vapor phase lateral diffusion from the mask region to the growth region, and hence a growth rate is increased beyond a growth rate in the case of non-selective growth.
Among the above-mentioned two mechanisms, the second one, namely vapor phase lateral diffusion is predominant. Accordingly, when a quantum well structure is to be formed by selective growth, a well layer would have a greater width with an increase in the mask width Wm, and thus the quantum well structure would have a smaller band-gap or a longer band-gap wavelength.
A compositional change occurs mainly due to a change in a crystal composition of group III source materials such as In and Ga in InGaAsP family materials. A change in a crystal composition is explained as follows. As earlier explained, selective growth occurs due to vapor phase lateral diffusion of source materials. In the vapor phase lateral diffusion, since a decomposition rate or a diffusion rate is different between In and Ga source materials, a concentration ratio between In and Ga varies during vapor phase from a mask region to a growth region. Accordingly, if a mask width were changed, a concentration ratio between In and Ga to be supplied to a growth region varies. Hence, when a quantum well structure including a well layer made of InGaAsP is grown, compressive strain is introduced into the quantum well structure due to an increase in a concentration ratio of In, and resultingly, the quantum well structure would have a smaller band-gap.
As discussed above, since a growth rate and composition of a quantum well structure varies in dependence on a mask width, a multi-quantum well structure having different thicknesses and band-gaps may be formed by common epitaxial growth by selectively growing a multi-quantum well structure made of InGaAsP family material with masks having widths which are different when measured in a stripe direction.
FIG. 6 illustrates curves showing the dependency of a growth-rate increasing rate on a mask width in MOVPE selective growth. The illustrated curves were obtained by the experiments the inventor had conducted using a growth pressure as a parameter. A growth-rate increasing rate is greater under a higher growth pressure. This phenomenon has been reported by the following persons.
(a) K. Tanabe et al., xe2x80x9cGrowth Pressure Dependence of MOVPE Selective Growthxe2x80x9d, Extended Abstracts of the 39th Spring Meeting 1992, The Japan Society of Applied Physics and Related Societies, 30a-SF-29, No. 3, pp. 976.
(b) Sasaki et al., Journal of Crystal Growth, Vol. 145, 1994, pp. 846-851
(c) T. Fujii, xe2x80x9cGrowth rate enhancement in selective area MOVPE based on vapor phase diffusion modelxe2x80x9d, Extended Abstracts of the 56th Autumn Meeting 1995, The Japan Society of Applied Physics, 28a-ZF-6, No. 1, pp. 293.
If a growth pressure were lowered, the dependency of a growth rate on a mask width as well as the dependency of a compositional change on a mask width is weakened, and as a result, a compositional change caused by a mask width, or a band-gap change caused by a mask width is made smaller.
FIG. 7 illustrates curves showing the dependency of a growth rate increasing rate on a mask width in MOVPE selective growth. The illustrated curves were obtained by the experiments the inventor conducted using group V source materials and a V/III ratio as parameters. When arsine and phosphine, V/III ratios of which are in the range of 20 to 1000, are employed as group V source materials, or when tertiarybutylarsine (hereinafter, referred to simply as xe2x80x9cTBAxe2x80x9d) and tertiarybutylphosphine (hereinafter, referred to simply as xe2x80x9cTBPxe2x80x9d) both having a small V/III ratio such as a V/III ratio=5 are employed, a growth rate increasing rate becomes greater. In contrast, when TBA and TBP having a great V/III ratio such as a V/III=100 or greater are employed, the dependency of a growth rate on a mask width is weakened. The similar results have been reported by Y Sakata et al., xe2x80x9cSelective MOVPE Growth of InGaAsP and InGaAs Using TBA and TBPxe2x80x9d, Journal of Electronic Materials, Vol. 25, No. 3, 1996, pp. 401-406.
When TBA and TBP having a great V/III ratio are used in MOVPE selective growth, the dependency of a growth rate on a mask width and further the dependency of a compositional change on a mask width are weakened. As a result, a compositional change caused by a change of a mask width, or a change in a band-gap caused by a change of a mask width is made smaller.
The problems accompanied with the conventional SSC-LDs can be solved by utilizing the above-mentioned the dependency of a growth rate and the dependency of group V source materials in MOVPE selective growth. Specifically, in a step of simultaneously forming a multi-quantum well layer and an SSC region in SSC-LD by MOVPE selective growth, a multi-quantum well layer is grown under a condition where a growth rate increasing rate is small, for instance, under a condition where a growth pressure is equal to or lower than 30 hPa, or TBA and TBP having a V/III ratio equal to 100 are used, whereas an SCH layer or a barrier layer is grown under a condition where a growth-rate increasing rate is great, for instance, under a condition where a growth pressure is equal to or higher than 200 hPa, or TBA and TBP having a V/III ratio equal to or smaller than 5 are used. This ensures a film thickness difference necessary for spot-size conversion with an almost constant band-gap all over a resonator.
This structure ensures an optical gain to a laser oscillation wavelength in overall range of a resonator, resulting in elimination of optical loss in an SSC region similarly to the conventional SSC-LDs. In addition, it is no longer necessary to form a region only for spot-size conversion, which ensures that a device length can be shortened equal to a length of a conventional semiconductor laser. Accordingly, a lower threshold current and a high temperature operation characteristic can be achieved, and a yield per a unit area or per a wafer can be significantly enhanced.
The above-mentioned structure and method of fabricating the same may be applied to an optical semiconductor modulator with SSC function (SSC-modulator), an optical semiconductor amplifier (SSC-amplifier), and a waveguide type pin photo-diode (SSC-pin-PD).
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.